Increasingly stringent federal regulations limit the permissible levels for emissions. As such, vehicle manufacturers have developed various methods to reduce emissions while improving vehicle performance and fuel economy. Catalytic converters are often used to reduce emission levels of regulated exhaust gases. The conversion efficiency of a catalytic converter may be monitored using a pre-catalyst oxygen sensor (HEGO sensor) positioned upstream from the catalytic converter and a post-catalyst oxygen sensor (catalyst monitor sensor or CMS) positioned downstream from the catalytic converter.
One method known for indicating conversion efficiency of the catalyst is to calculate a ratio of the accumulated number of CMS transitions or switches to the accumulated number of HEGO transitions or switches. An increasing switch ratio is generally indicative of a degrading catalyst. When the switch ratio exceeds a threshold value, a malfunction indicator light (MIL) is illuminated to alert the vehicle operator.
Another known method for indicating conversion efficiency of the catalyst determines a ratio based on an accumulated per sample change in magnitude of CMS sensor voltage relative to a corresponding change in magnitude of the HEGO sensor voltage.